This invention relates to a circuit configuration for voltage supply of a two-wire sensor which is connected to a supply voltage source via a first connection line, in which a voltage longitudinal controller is positioned, and via a second connection line, in which the controller output is connected to the second connection line via a series circuit comprising at least two oppositely poled limiting diodes, and in which a current-limiting resistor is positioned in one of the two connection lines between the two-wire sensor and the series circuit comprising the limiting diodes.
A two-wire sensor is developed from a sensor element which measures a physical dimension, such as temperature, pressure or field strength of a magnetic field, as well as electronic components for processing signals supplied by the sensor element. Both the power supply and the conduction of the measured and processed measuring signals is effected only via two lines from which the name two-wire sensor is derived. Therefore, a two-wire sensor only has two connections which simultaneously serve to supply power and to conduct the measured and processed measuring signals.
Two-wire sensors, which are used in explosion-endangered areas, are equipped with a circuit configuration for current and voltage limitingxe2x80x94a so-called electrical barrierxe2x80x94to prevent explosions. To protect against overvoltages which might trigger potential ignition, the two connection lines are interconnected via at least one limiting diode. To protect the sensor against excessive currents which would cause the sensor to overheat, a series resistor is installed in the line after the limiting diode. To protect the limiting diode against excessive high voltages, a voltage longitudinal controller, for example, is fit in the connection line, the output voltage of which during operation must always be smaller than the limiting voltage of the limiting diode.
Two-wire sensors, which are fitted with a so-called HART(copyright) interface, however, require a so-called HART(copyright) resistor of a prescribed value in one of the two connection lines leading from the supply voltage to the two-wire sensor. With a severely loaded two-wire sensor, this additional longitudinal resistor in one of the two connection lines may result in the supply voltage no longer being sufficient for flawless operation of the two-wire sensor.
Therefore, it is the object of the present invention to design a circuit arrangement for a voltage supply of a two-wire sensor such that, on the one hand, overvoltages and overcurrents are avoided with certainty, while, on the other hand, a sufficient supply voltage for flawless operation is always provided.
The foregoing object is achieved in the present invention by means of the features specified in claim 1, in that the joint connection point of the two limiting diodes is connected to the control input of the voltage longitudinal controller.
The inventive measure, obtaining the regulating quantity for the voltage longitudinal controller from a pick-off of the series circuit of the limiting diodes acting as a voltage divider, has the advantage that the limiting voltage can be selected considerably smaller than with prior art electrical barriers. Because of the low limiting voltage, the longitudinal resistors, above all, the current-limiting resistor, in the connection lines from the supply voltage to the two-wire sensor can also be dimensioned smaller, so that the voltage lossxe2x80x94i.e., the voltage drop at the longitudinal resistorsxe2x80x94caused by the supply current for the two-wire sensor is also reduced. Because of low voltage losses, the inventive circuit configuration, therefore, generates a higher supply voltage than known electric barriers for the two-wire sensor, in spite of equal supply voltages.
The invention is explained in detail by means of the figures.